BAGHDAD, Iraq (AP) - A U.S. Marine transport helicopter crashed in flames Wednesday in a field northwest of Baghdad, killing all seven people aboard, the U.S. military said. It was the fifth U.S. aircraft lost in less than three weeks and the latest sign of growing problems with aviation in Iraq.

A U.S. military statement gave no reason for the crash of the CH-46 Sea Knight, which went down near Fallujah in Anbar province, about 20 miles from Baghdad. However, at the Pentagon, three Marine Corps officials said the troop-transport helicopter was in flames when it went down, with the pilot appearing to attempt a hasty landing but losing control as the aircraft descended.

They said witnesses in nearby Marine aircraft saw the flames but saw no sign that it involved hostile fire.

An Iraqi air force officer, however, said the helicopter was downed by an anti-aircraft missile. He spoke on condition of anonymity because he was not authorized to release information.

An Iraqi farmer who lives about a half mile from the crash site said he heard a missile fired moments before the crash, which took place in an insurgent-infested region.